1. Field of the Invention
The present invention relates to an image processing apparatus employing a spatial light modulator (termed as light modulator). More particularly, the invention relates to a color image processing apparatus which can input and store an image on a two-dimensional light modulator, process the image, and write the processed image on a paper.
2. Description of a Related Art
As a typical image processing apparatus, there has been known a color copying machine for creating, editing and copying a color image. This color copying machine can be divided into two types, that is, an analog type and a digital type, each of which has its own advantages and disadvantages.
The analog type color copying machine is inferior to the digital type machine in image processing such as color correction or tone processing, because the former machine directly uses an optical image for exposure and development in creating a color copy. On the other hand, the latter machine can more easily implement the image processing, because it takes the steps of converting an original image into a digital signal with an image scanner and creating a color copy based on the digital signal with a printer.
Herein, the term "image processing" means a wide range of processing including tone processing (gamma correction, shading correction), sharpness intensity, area specification (trimming, masking), color processing (color reproduction, painting function, color cutting), movement (rotation), and edition (insert-synthesizing, character synthesizing).
In comparing the color copies created by both machines with each other, the analog type color copying machine offers smooth image quality (the resolution is about 800 DPI), however it has difficulty in implementing a color masking function (separation of black from chromatic colors), so that it cannot offer excellent color reproduction. Concretely, a dark line often appears on the color copy. On the other hand, the digital type machine can easily implement color masking, so that it is superior in color reproduction, however, it has difficulty in fining the copy scale more. Though it may have more pixels for fining the copy scale more, the digital type machine has to handle more quantity of data so that it may require a high-speed processor and a large-capacity memory. This means that the digital type machine is far more costly than the analog type one.
It will be apparent from the above description that the analog type color copying machine is superior in copying a large number of papers, while the digital type machine is superior in color reproduction. It is thus suitable for fine graphic materials or printing a small number of copies.
The foregoing compared results between the analog type color copying machine and the digital type one are summarized in Table 1.
TABLE 1 ______________________________________ Analog Type Digital Type ______________________________________ Color Reproduction Normal Excellent Black Reproduction Normal Excellent Graininess Excellent Normal Character Reproduction Excellent Normal (Smoothness) ______________________________________
As mentioned above, as the analog and digital type copying machines have their respective advantages and disadvantages, these machines are used for the proper graphic materials (pictures, writing, printing, etc.), which are respectively determined on a copy kind, a purpose and way of use.
Traditionally, however, there has been provided no apparatus functioning as both the analog and the digital type copying machines. For using a right machine for a right graphic material, it has been necessary to prepare at least two machines.
Moreover, there has been traditionally provided no apparatus which is capable of realizing both of the superior processings in the analog type and the digital type machines. Hence, it has been necessary to employ an expensive digital type copying machine for creating a color copy on which the high-fining quality and excellent color reproduction are realized.
Traditionally, no apparatus has been provided which an analog image is mixed with a digital image on one display.
Further, a known color copying machine, though it is a digital type or analog type, has the restriction that the scanning density of a light beam depends on the image-writing or -reading speed or the image quality. That is, as the scanning density becomes more coarse, the image quality becomes inferior but the scanning and transmission time becomes shorter, because the information quantity is reduced. For obtaining an expanded image, it is possible to read an image at a fine scanning density and write it at a normal scanning density.
Traditionally, however, the known color copying machine has a shortcoming that the diameter of a laser beam spot restricts the scanning density. That is, the fine scanning density is disallowed to be more fine than the beam spot. It results in dwarfing improvement of image quality.
Conversely, the small beam spot is disallowed to exactly represent an image formed at coarse scanning density. In particular, the small beam spot brings about a large error in reading a pattern having a spacial frequency close to the scanning density.
A known image processing apparatus suffers from another shortcoming that each graphic material has the corresponding quantity of reflected light because of the paper quality and color when analogously writing the image reflected from the graphic material. The difference of the reflected light of the graphic material makes it quite difficult to constantly record an excellent image with high contrast in the light modulator. As a result, it is difficult to constantly output a high-quality image.
Another shortcoming the normal image processing apparatus may suffer from is ununiformity brightness with which an image is spatially written in the light modulator. The ununiformity brightness results from two-dimensional ununiformity of lightning to a subject graphic material, variations of an optical characteristic of an image-forming lens system and a characteristic of a light modulator, and the like.
Further, a known color-image processing apparatus has been traditionally required to repeat the process of analogously writing an image in the light modulator, digitally reading it out of the light modulator, processing it, digitally rewriting it in the light modulator, and analogously reading it out of the light modulator with respect to a red-, a green-, and a blue-component colors. The repetition of the process results in increasing dead processing and lowering the processing speed. Further, the known color image processing apparatus is uncapable of reading an image of a moving object.
Turning to another type image processing apparatus, herein, an image input device can be considered. The most common one is a linear image sensor. The linear image sensor includes a pixel row functioned as main scanning, an image sensor and an optical system to be mechanically moved as sub scanning with respect to a subject graphic material. Both main scanning and sub scanning results in obtaining image information.
The linear image sensor is divided into two types, that is, a reduction type image sensor and a contact type image sensor.
The reduction type image sensor is a sensor which includes a lens and a charge coupled device (CCD) and serves to reduce a subject graphic material through a lens and read the reduced image with the CCD. The contact type image sensor, on the other hand, is an equal-magnification sensor designed to have a longer length than the width of the subject graphic material. As the contact type, there have been provided a multi-chip type image sensor employing CCDs, a film type image sensor employing the combination of a film type image sensor and a lens array, a closely-contact type image sensor allowing the film type image sensor to directly contact the subject graphic material without using the lens array, and the like. The contact image sensor has some advantages such as small-size and simple construction of the optical system. Due to these advantages, the demand for the contact type has been rising.
A color image input device basically employs a linear image sensor. As the linear color image sensor, there have been provided a color-filter type image sensor and a three-light-source type color image sensor, for example. The former image sensor provides three color filters located in sequence before the pixel row. The three colors are R (red), G(green) and B(blue) or yellow, cyan and magenta. The latter sensor is designed to repeat the main scanning done by the linear image sensor three times for each emission of three colors R, G and G. The color-filter type image sensor can offer just one-third time as low resolution as the monochrome image sensor, while the three-light-source type color image sensor needs three times as long a reading time as the monochrome image sensor.
Recently, however, the color image input device has been requested to enhance the resolution and scanning speed.
To improve the resolution of the linear image sensor, it is necessary to make the photoelectric elements more dense. As the photoelectric element becomes more dense, the area per one pixel becomes smaller, the output signal becomes lower, and the sensitivity becomes inferior. The longer reading time of a signal from the photoelectric elements may compensate for the inferior sensitivity. However, the longer reading time is added to a delay caused by the photoelectric elements increased as a result of densifying them, resulting in remarkably lowering the image scanning speed. Further, the main-scanning linear image sensor having more integrated photoelectric elements requires higher precision and scanning speed for the sub-scanning mechanical driving system. Today, the precision and the scanning speed have substantially reached its limit. Besides, it is difficult to improve the performance of the mechanical driving system.
As mentioned above, the linear color image sensor needs higher resolution and an improved scanning time if it reaches the same capability as the monochrome image sensor.